Servlets :: Form Based JAAS - How To Implement Login System Explicitly Outside Container

I have configured form based JAAS in my app. Basically, in web.xml I have declared security constraints on certain jsp page, declared specific roles, login and error pages. So, my login form is:

<form id="loginForm" action="j_security_check" method="post">
<p>
<label for="name">Username</label> <input name="j_username"
id="username" type="text" required/>

[Code] ....

This works fine when some user tries to access some of the pages declared in <security-constraint> tag of web.xml.

Container automatically manages login process, redirects to login page and if login details are valid, gives access to secured page.

Now, how should I implement login system so that user can go to login page (possibly same login form) and log in from there?